1. Field of the Invention
The invention described herein relates to a laminated composite of a sialon layer bonded to a surface of a material, more particularly, this material is a refractory material such as silicon nitride or graphite. In addition the invention relates to a method for forming the composite by chemical vapor disposition of the sialon. In particular, sialons are compositions which include all of the following: silicon, aluminum, oxygen, and nitrogen and are illustrated by Si.sub.3 Al.sub.3 O.sub.3 N.sub.5.
2. Description of the Prior Art
Efforts have been made to construct a helical expander to recover heat energy by directly expanding combustion gases in coal-burning power plants. It is expected, that if a suitable expander were available, the typical generating efficiency of these plants could be increased from 34-40% to 50-60%.
These efforts have not succeeded because the materials considered for constructing the expander could not survive the environment for a practical length of time. Fouling as a result of silica carried in the combustion gases is a major problem. Efforts at removing the silica prior to the gases entering the expander have not ameliorated the problem. The gases from the combustion are typically at temperatures between 1000.degree. C. and 1400.degree. C. and at a pressure on the order of 240 MPa (megapascals). Further, the gases include corrosives, such as, hydrogen sulfide and sulfuric acid.
Helical expanders having a ceramic rotor have been considered. The ceramic would provide protection against temperature and corrosives. Silicon nitride (Si.sub.3 N.sub.4), a ceramic which has been employed in stator blades of high-temperature gas turbines, has been suggested as the ceramic from which to fabricate parts for the helical expander.
Unfortunately, the compatibility of silicon nitride is not favorable to long term exposure to a combustion-gas environment. Rapid fouling and oxidation occurs and appears to be the result of the porosity and composition of the surface of parts fabricated with silicon nitride.